1. Field of the Invention
The present invention relates to a method for producing a high toughness hot rolled high tensile steel plate by a low temperature rolling. More particularly, the present invention relates to a method for producing a high toughness and high tensile steel plate having excellent low-temperature toughness which is suitable for use in cold districts, and has as one of its objects to provide a method for producing a high toughness and high tensile steel plate suitable for production of electric resistance welded pipe and tubes, spiral pipes and UO pipe for pipe lines which are required to have low-temperature toughness in particular.
2. Description of the Prior Art
As for the conventional methods for producing a steel plate for pipe lines usuable in low-temperature districts, following two types of methods are known.
According to the first type of the conventional methods, a steel slab or ingot containing precipitation hardening elements, such as, Nb, Ti and V is heated to a temperature not lower than the solid solution temperature of these elements, and is then rolled with heavy reduction in a finishing temperature range to obtain a fine precipitation hardened steel. This method is commonly called "a controlled rolling method" (hereinafter abridged as CR method).
The second type of the conventional methods is directed to production of a quenched and tempered high tensile steel plate based on the microstructure transformation and precipitation by quenching and tempering.
In the case of the first type of the conventional methods, although the transition temperature is lowered by the so-called delamination phenomenon in which laminar crackings occur in parallel to the direction of crack propagation on the fracture of an impact test piece, the absorbed energy is remarkably lowered so that the steel plate produced by this method has been regarded to be of no use in applications where high energy is required at low temperatures.
Therefore, it has been long sought for to improve the absorbed energy by the controlled rolling (CR) of a niobium, titanium or vanadium containing steel.
However, in the case of the second type of the conventional methods, it is possible to assure high absorbed energy because of the freedom from the occurrence of delamination in the impact test piece as is confronted in the first type of the methods. However, this method requires separate heat treating equipment, thus causing an increase in the production cost. In the case of the production of thin-gauge steel plates, particularly those thinner than 20 mm, this method has a remarkable disadvantage with respect to the product having a flat shape, for example, as compared with the non-quenched and non-tempered steel plate.
Now it is generally known that the impact value of a steel plate shows a far better value in the direction parallel to the rolling direction (L direction) than in the direction transverse to the rolling direction (T direction). This results from the difference between the rolling reductions in the L and T directions from the slab stage or ingot to the final-shape stage. In the case of a hot rolled steel strip which has been rolled mainly in the longitudinal direction of the steel slab or ingot with almost no rolling in the axial direction of the steel slab or ingot, the ratio of the impact value in the L direction to that in the T direction reaches about 3.
However, electric resistance welded pipes and tubes, etc., for pipe lines as mentioned hereinbefore are required to have toughness in the T direction in practical use.
Therefore, the present invention provides a method for economically producing a hot rolled steel plate, for example, which has excellent low-temperature toughness satisfying the above rquirements.
As one of the factors which lower the impact value in the T direction, there may be mentioned the presence of non-metallic inclusions which are composed mainly of manganese sulfides elongated in the rolling direction and cause notch effects on the test piece in the T direction, and thus elimination of the inclusions is very effective means for improving the impact value. Therefore, as a means for improving the impact value in the T direction, it has been conventionally proposed to add one or more of the rare earth metals, such as, La, Ce and Pd which convert MnS into a sulfide having little plasticity at the ordinary hot rolling temperature.
Meanwhile, as a value for indicating toughness the fracture appearance transition temperature is used other than the above impact value, and a lower fracture appearance transition temperature is directly related to a better low temperature toughness. However it has been confirmed by many researchers, for example, by studies of Brownrigg published in JISI Vol. 208, page 1078 (1970), that when one or more of the rare earth metals (hereinafter referred as REM), such as, La, Ce and Pr, is added for the purpose of improving the impact value as mentioned before, the fracture appearance transition temperature rises in some cases, and thus it has been known that REM can not always be added without restriction as the means for improving the toughness.
Furthermore, as an effective means for improving the low temperature toughness, heat treatment, such as, quenching and tempering, have been known and widely used for high-grade heavy steel plates, but such heat treatments present many difficulties in both the technical and economical aspects.
Also, it has been known to lower the transition temperature by means, such as, low temperature rolling and controlled rolling for the purpose of grain refining, but these means have been found not to contribute substantially to improvements in the shelf energy.
The present inventors have conducted studies for a many years on the production of a high tensile steel having excellent low temperature toughness, and have succeeded in clarifying the reason why REM addition raises the transition temperature and deteriorates the low temperature toughness and have discovered that, not only is the impact value improved over the whole temperature range, but also the transition temperature is lowered under the as rolled condition if the steel component elements including REM and S are specified and the rolling is done under certain conditions. The present invention is based on the above discovery.